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Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores

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On Thu, Feb 25, 2016 at 06:07:03AM -0800, Paul E. McKenney wrote:
> Still bad wording...
> 
> It hasn't actually moved anything over the barrier().  It has instead
> moved both the barrier() and the WRITE_ONCE(b, p) to precede the "if
> (q)".  Mathieu mentioned this over IRC yesterday, and I queue a change
> so that the paragraph now reads as follows:
> 
>   (*) If both legs of the "if" statement begin with identical stores to
>       the same variable, then those stores must be ordered, either by
>       preceding both of them with smp_mb() or by using smp_store_release()
>       to carry out the stores.  Please note that it is -not- sufficient
>       to use barrier() at beginning of each leg of the "if" statement
>       because, as shown by the example above, optimizing compilers can
>       destroy the control dependency while respecting the letter of the
>       barrier() law.
> 
> Does hat help?

Maybe.. I still feel the compiler should not do this; but I'm having a
hard time explaining why.

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[PATCH tip/core/rcu 0/14] Documentation updates for 4.6 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 06:10 +0100
  [PATCH tip/core/rcu 01/14] documentation:  Add real-time requirements from CPU-bound workloads "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 06:10 +0100
  [PATCH tip/core/rcu 14/14] documentation: Clarify compiler store-fusion example "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 06:10 +0100
  [PATCH tip/core/rcu 12/14] documentation: Document illegality of call_rcu() from offline CPUs "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 06:10 +0100
  [PATCH tip/core/rcu 07/14] documentation: Distinguish between local and global transitivity "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 06:10 +0100
  [PATCH tip/core/rcu 05/14] documentation: Remove obsolete reference to RCU-protected indexes "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 06:10 +0100
  [PATCH tip/core/rcu 08/14] documentation:  Add alternative release-acquire outcome "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 06:10 +0100
  [PATCH tip/core/rcu 11/14] documentation: Transitivity is not cumulativity "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 06:10 +0100
  [PATCH tip/core/rcu 04/14] documentation: Add synchronize_rcu_mult() to the requirements "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 06:10 +0100
  [PATCH tip/core/rcu 13/14] documentation: Explain how RCU's combining tree fights contention "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 06:20 +0100
  [PATCH tip/core/rcu 10/14] documentation: Explain why rcu_read_lock() needs no barrier() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 06:20 +0100
  [PATCH tip/core/rcu 03/14] documentation: Fix memory-barriers.txt section references "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 06:20 +0100
  [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 06:20 +0100
    Re: [PATCH tip/core/rcu 02/14] documentation: Fix control  dependency and identical stores Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2016-02-24 22:20 +0100
      Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency  and identical stores "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 22:50 +0100
        Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency  and identical stores Jianyu Zhan <nasa4836@gmail.com> - 2016-02-25 07:50 +0100
          Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency  and identical stores "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-25 15:10 +0100
        Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency  and identical stores Peter Zijlstra <peterz@infradead.org> - 2016-02-25 09:30 +0100
          Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency  and identical stores "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-25 15:10 +0100
            Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency  and identical stores Peter Zijlstra <peterz@infradead.org> - 2016-02-25 15:50 +0100
              Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency  and identical stores "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-25 16:50 +0100
  [PATCH tip/core/rcu 06/14] documentation: Subsequent writes ordered by rcu_dereference() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-02-24 06:20 +0100

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